The research conducted for this thesis investigates the impact of thesauri on users' information searching behaviour, and the integration of thesauri into information searching interfaces in order to support the searching process. The work reported here consists of two preliminary studies, which served to refine research questions and explore the methodology, followed by two in-depth studies. The two in-depth studies were conducted with FACET, an experimental system developed at the University of Glamorgan in the context of an EPSRC (Engineering and Physical Sciences Research Council)-funded project focussing on faceted information retrieval in indexed multimedia collections. These studies were conducted with the participation of museum and library professionals. The author used a combination of qualitative and quantitative methods, such as "think aloud" protocols, questionnaires, application log files and content analysis. The in-depth studies resulted in important findings regarding the FACET interface which were considered in its further development. Findings also relate to thesaurus use in general - it was for example observed that behaviours such as browsing the thesaurus are not suitable at all search stages, so that users need to be guided in their choice of tools. One of the main findings was that conceptual problems encountered by searchers with little formal search training caused more difficulties than those related to interface design. This resulted in the conclusion that the information searching process needs to be supported by the interface and that more extensive use of thesauri in the form of support tools is possible at different stages of the search process. Based on literature on the information searching process and data from the first FACET study, the basis for a model of information searching in controlled vocabulary enhanced systems was developed and subsequently refined with data from the second in-depth study. This model aims in particular at facilitating the design and development of such systems. It consists of a textual and graphical representation of the search stages and an account of potential problems, their causes, possible detrimental effects on the further progress of the search and suggestions on how these can be avoided, under particular consideration of tools based on controlled vocabularies.